Electrodes for use in industrial electrolysis cells must generally meet a combination of strict requirements with regard to conductivity, physical and chemical stability, corrosion resistance, manufacture and electrochemical performance, more particularly catalytic activity and selectively.
However, there is no known material which can meet all of these requirements for satisfactory performance of industrial electrodes. The very few materials which are able to withstand severe anodic attack can generally not be used alone to produce electrodes with adequate electrochemical performance under industrial operating conditions. Consequently, various types of composite electrodes comprising different combinations of materials have been proposed, in order to be able to meet as far as possible the various technical and economic requirements for providing adequate industrial performance.
Various types of electrodes comprising a catalytic coating on a metal base have been proposed, as may be seen from the numerous patents relating to such electrode coatings.
An outstanding success in this field is the dimensionally stable anode, known under the tradename DSA and described e.g. in U.S. Pat. No. 3 632 498, which comprises a catalytic coating consisting of titanium-ruthenium oxide formed on a titanium base, and which has fundamentally changed the chlorine industry throughout the world in the past decade.
An electrode base of titanium is preferred because titanium and other suitable valve metals can exhibit extremely high corrosion resistance due to their film forming properties whereby a protective oxide film is formed under anodic operating conditions.
Platinum group metals are known to provide excellent electrocatalysts for different electrode reactions but their high cost makes it necessary to use them as sparingly as possible, and more particularly to replace them by cheaper electrode materials whenever possible. Ruthenium is of particular interest due to its relatively low cost and availability with respect to the other platinum group metals.
The dimensionally stable anode (DSA) mentioned above exhibits excellent, stable performance with a long service life in chlorine production cells. This DSA must, however, be manufactured and operated under controlled conditions in order to avoid the formation of an insulating titanium oxide layer on the electrode base, which would result in electrochemical passivation of the anode with an excessive rise of its operating potential.
Another anode, as described e.g. in U.S. Pat. No. 3 776 834 comprises a catalytic coating with tin replacing about one half of the ruthenium normally contained in the standard coating of the titanium-ruthenium oxide of said DSA. This anode with partial replacement of ruthenium by tin exhibits a higher oxygen overvoltage and an improved resistance to oxidation in presence of anodically generated oxygen than thhe standard DSA currently used in the chlor-alkali industry.
Various inexpensive electrode materials based on non-noble metals have been proposed but their use has nevertheless remained relatively restricted for various reasons.
Lead dioxide is also a promising stable, inexpensive anode material for various processes, but massive lead dioxide anodes exhibit inadequate conductivity. On the other hand, lead dioxide coatings formed on an electrode base have generally not provided satisfactory stable performance with a high service life in industrial operation. The state of the art relating to lead dioxide electrodes, their manufacture, and use, may be illustrated by U.S. Pat. Nos. 4 040 039, 4 026 786, 4 008 144, 3 751 301, 3 629 007 and U.K. Pat. Nos. 1 416 162, 1 378 884, 1 377 681.
Manganese dioxide also shows great promise as a stable, inexpensive anode material, especially for oxygen evolution in processes for electrowinning metals from acid solutions. Its widespread use has nevertheless been hindered hitherto by manufacturing difficulties: the manufacture of satisfactory massive electrodes consisting entirely of manganese dioxide has not been possible, while manganese dioxide coatings formed on an electrode base have generally not provided satisfactory stable performance with a high industrial service life.
Lead dioxide and manganese dioxide coatings may be produced by thermal decomposition of metal salts deposited on the electrode base forming the coating substrate, but the resulting oxide coating is nevertheless generally quite porous and has poor adherence to the base. On the other hand, more compact oxide coatings with better adherence may be produced by electrodeposition on the electrode base, but they are nevertheless porous and generally still provide inadequate protection of the electrode base from oxidation.
It has moreover been proposed to provide the metal electrode base with an intermediate protective coating which is covered with an outer coating of lead or manganese dioxide. The state of the art relating to such intermediate protective coatings may be illustrated by U.S. Pat. Nos. 4 028 215, 4 125 449, 4 040 937 (Sn/Sb oxide subcoating); Japanese Patent Application No. 51-156740, publication No. 53-79771 and Electrochimica Acta Vol. 23, p. 331-333 (Pt Group metal oxide subcoating); U.S. Pat. No. 4 072 586 (RuO.sub.2 /TiO.sub.2 subcoating); U.S. Pat. No. 4 180 445 (TiO.sub.2 /SnO.sub.2 /RuO.sub.2 subcoating); and U.S. Pat. No. 4 060 476 (TiN subcoating).
Such intermediate protective coatings must form an effective barrier against oxidation of the electrode base and must meet various requirements for this purpose with regard to adherence, conductivity, cost, impermeability, resistance to oxidation, physical and chemical stability. This particular combination of properties is nevertheless difficult to achieve in industrial practice.
Various proposals have also been made to use polymeric materials in the production of electrodes. Thus, for example, according to U.S. Pat. No. Reissue 29419, a catalytic composite coating formed on a valve metal base, comprises ruthenium dioxide finely dispersed in an organic polymer intended to serve as a binder for mechanical support of the dispersed electrocatalyst, adhesion to the underlying base, and protection thereof. The ruthenium dioxide is prepared in the form of extremely fine particles of less than 0.1 micron size and uniformly dispersed in the polymer in a weight ratio of 6:1 to 1:1 to provide the electrical and catalytic properties of the coating. The conductivity of such a composite coating will thus depend essentially on the amount of dispersed electrocatalyst, on its particle size and on its distribution in the polymer (binder). The state of the art relating to electrodes comprising polymeric materials may further be illustrated by U.S. Pat. Nos. 3 626 077, 3 751 301, 4 118 294, 3 972 732, 3 881 957, 4 090 979 and the laid-open German Patent Application, Offenlegungsschrift No. 2 035 918.
The service life of coated electrodes such as those mentioned above is nevertheless generally limited when they are operated industrially in presence of a notable anodic generation of oxygen. A particular problem in this connection is that of ensuring adequate protection of the electrode base from attack by oxidation leading to electrode failure due to corrosion or electrochemical passivation of the base.
It may thus be seen from the foregoing that, in addition to the choice of suitable electrode materials, the production of electrodes with satisfactory, long-term performance in industrial electrolytic processes is generally quite problematic and presents complex technological problems.